Showing papers on "Near and far field published in 2003"

Microstrip antennas integrated with electromagnetic band-gap (EBG) structures: a low mutual coupling design for array applications

Abstract: Utilization of electromagnetic band-gap (EBG) structures is becoming attractive in the electromagnetic and antenna community. In this paper, a mushroom-like EBG structure is analyzed using the finite-difference time-domain (FDTD) method. Its band-gap feature of surface-wave suppression is demonstrated by exhibiting the near field distributions of the electromagnetic waves. The mutual coupling of microstrip antennas is parametrically investigated, including both the E and H coupling directions, different substrate thickness, and various dielectric constants. It is observed that the E-plane coupled microstrip antenna array on a thick and high permittivity substrate has a strong mutual coupling due to the pronounced surface waves. Therefore, an EBG structure is inserted between array elements to reduce the mutual coupling. This idea has been verified by both the FDTD simulations and experimental results. As a result, a significant 8 dB mutual coupling reduction is noticed from the measurements.

Averaged transition conditions for electromagnetic fields at a metafilm

Abstract: This paper derives generalized sheet transition conditions (GSTCs) for the average electromagnetic fields across a surface distribution of electrically small scatterers characterized by electric and magnetic polarization densities. We call such an arrangement of scatterers a metafilm-the two-dimensional (2-D) equivalent of a metamaterial. The derivation is based on a replacement of the discrete distribution of scatterers by a continuous one, resulting in a continuous distribution of electric and magnetic polarization densities in the surface. This is done in a manner analogous to the Clausius-Mossotti-Lorenz-Lorentz procedure for determining the dielectric constant of a volume distribution of small scatterers. The result contains as special cases many particular ones found throughout the literature. The GSTCs are expected to have wide application to the design and analysis of antennas, reflectors, and other devices where controllable scatterers are used to form a "smart" surface.

Near-Field Second-Harmonic Generation Induced by Local Field Enhancement

Abstract: The field near a sharp metal tip can be strongly enhanced if irradiated with an optical field polarized along the tip axis. We demonstrate that the enhanced field gives rise to local second-harmonic (SH) generation at the tip surface thereby creating a highly confined photon source. A theoretical model for the excitation and emission of SH radiation at the tip is developed and it is found that this source can be represented by a single on-axis oscillating dipole. The model is experimentally verified by imaging the spatial field distribution of strongly focused laser modes.

Near-field fluorescence microscopy based on two-photon excitation with metal tips

Abstract: We present a new scheme for near-field fluorescence imaging using a metal tip illuminated with femtosecond laser pulses of proper polarization. The strongly enhanced electric field at the metal tip ( $\ensuremath{\approx}15\mathrm{nm}$ end diameter) results in a localized excitation source for molecular fluorescence. Excitation of the sample via two-photon absorption provides good image contrast due to the quadratic intensity dependence. The spatial resolution is shown to be better than that of the conventional aperture technique. We used the technique to image fragments of photosynthetic membranes, as well as $J$-aggregates with spatial resolutions on the order of 20 nm.

Surface modes for near field thermophotovoltaics

Abstract: Thermal radiative energy transfer between closely spaced surfaces has been analyzed in the past and shown not to obey the laws of classical radiation heat transfer owing to evanescent waves and, more recently, electromagnetic surface modes. We have analyzed the energy transfer between layered media, one of the layers being the thermal source, using a Green’s functions method and the fluctuation-dissipation theorem. Based on the analysis, we propose a structure that can utilize the surface modes to increase the power density and efficiency of low temperature thermophotovoltaic generators.

Imaging the near field

Abstract: In an earlier paper we introduced the concept of the perfect lens which focuses both near and far electromagnetic fields, hence attaining perfect resolution. Here we consider refinements of the original prescription designed to overcome the limitations of imperfect materials. In particular we show that a multilayer stack of positive- and negative-refractive-index media is less sensitive to imperfections. It has the novel property of behaving like a fibre-optic bundle but one that acts on the near field, and not just the radiative component. The effects of retardation are included and minimized by making the slabs thinner. Absorption then dominates image resolution in the near field. The deleterious effects of absorption in the metal are reduced for thinner layers.

Near-field optical transducers for thermal assisted magnetic and optical data storage

Abstract: An optical transducer comprises an optical element for directing an electromagnetic wave to a focal region and a metallic nano-structure having a longitudinal axis substantially parallel to an electric field of the electromagnetic wave, the metallic nano-structure being positioned outside of the optical element, wherein the electromagnetic wave produces surface plasmons on the metallic nano-structure. A cladding material having a refractive index differing from the refractive index of the optical element can be positioned adjacent to a surface of the metallic nano-structure. Magneto-optical recording heads that include the transducers and disc drives that include the magneto-optical recording heads are also included.

Numerical analysis of stacked dielectric resonator antennas excited by a coaxial probe for wideband applications

Abstract: The objective of the present study is to improve the bandwidth of the dielectric resonator antenna (DRA) excited by a coaxial probe by using a stacked DRA configuration above an infinite ground plane. The DRA is axisymmetric and a coaxial probe is placed off the antenna axis to excite the HEM/sub 11/spl delta// mode in the DRA, resulting in a broadside radiation pattern. A surface integral equation formulation and the method of moments are used for the numerical analysis. The input impedance and the far field radiation patterns have been computed and the effects of different parameters on the antenna performance have been investigated. With the proper excitation and selection of the resonator parameters, a bandwidth of 35% has been achieved for the stacked DRA configuration based on a -10 dB reflection coefficient on a 50 /spl Omega/-transmission line. An equivalent circuit model is postulated to describe the dual-resonance behavior of the stacked antenna system.

Plasmon-coupled tip-enhanced near-field optical microscopy

Abstract: Near the cut-off radius of a guided waveguide mode of a metal-coated glass fibre tip it is possible to couple radiation to surface plasmons propagating on the outside surface of the metal coating. These surface plasmons converge toward the apex of the tip and interfere constructively for particular polarization states of the initial waveguide mode. Calculations show that a radially polarized waveguide mode can create a strong field enhancement localized at the apex of the tip. The highly localized enhanced field forms a nanoscale optical near-field source.

Performance of visible and mid-infrared scattering-type near-field optical microscopes.

Abstract: We describe the principles of two scattering-type near-field optical microscopes (s-SNOMs), one operating at 633 nm wavelength, the other at selectable wavelengths in the range 7.3-11.3 micro m, and compare the measurement experience. Both use interferometric detection of scattered radiation, and are therefore capable of amplitude and phase-contrast imaging. In this study both instruments use the same or even identical commercial probe tips, and measure a single, three-component, test sample. Our results show that the imaging process of s-SNOM is wavelength-independent, namely, that the resolution is determined by the properties of the tip only, and that the contrast is given by the complex refractive index of the sample, predictable from a simple, analytical model of tip-sample interaction. A novel, 'edge-darkening' artefact is described which may appear in s-SNOM and that is wavelength-independent.

Non-linear interferometric vibrational imaging

Abstract: A method of examining a sample, which includes: exposing a reference to a first set of electromagnetic radiation, to form a second set of electromagnetic radiation scattered from the reference; exposing a sample to a third set of electromagnetic radiation to form a fourth set of electromagnetic radiation scattered from the sample; and interfering the second set of electromagnetic radiation and the fourth set of electromagnetic radiation. The first set and the third set of electromagnetic radiation are generated from a source; at least a portion of the second set of electromagnetic radiation is of a frequency different from that of the first set of electromagnetic radiation; and at least a portion of the fourth set of electromagnetic radiation is of a frequency different from that of the third set of electromagnetic radiation.

Electric fields that "arrive" before the time derivative of the magnetic field prior to major earthquakes.

Abstract: The low frequency electric signals (emitted from the focal area when the stress reaches a critical value) that precede major earthquakes, are recorded at distances approximately 100 km being accompanied by magnetic field variations. The electric field "arrives" 1 to 2 s before the time derivative of the horizontal magnetic field. An explanation, which is still awaiting, should consider, beyond criticality, the large spatial scale as well as that the transmission of the electromagnetic fields (through an inhomogeneous weakly conductive medium like the Earth) obeys diffusion type equations.

Apertureless near-field optical microscopy: A study of the local tip field enhancement using photosensitive azobenzene-containing films

Abstract: The local optical field enhancement which can occur at the end of a nanometer-size metallic tip has given rise to both increasing interest and numerous theoretical works on near-field optical microscopy. In this article we report direct experimental observation of this effect and present an extensive study of the parameters involved. Our approach consists in making a “snapshot” of the spatial distribution of the optical intensity in the vicinity of the probe end using photosensitive azobenzene-containing films. This distribution is coded by optically induced surface topography which is characterized in situ by atomic force microscopy using the same probe. We perform an extensive analysis of the influence of several experimental parameters. The results are analyzed as a function of the illumination parameters (features of the incident laser beam, exposure time, illumination geometry) as well as the average tip-to-sample distance and tip geometry. The results obtained provide substantial information about t...

Focal spots of size λ/23 open up far-field florescence microscopy at 33 nm axial resolution

Abstract: We report spots of excited molecules of 33 nm width created with focused light of lambda = 760 nm wavelength and conventional optics along the optic axis. This is accomplished by exciting the molecules with a femtosecond pulse and subsequent depletion of their excited state with red-shifted, picosecond-pulsed, counterpropagating, coherent light fields. The lambda/23 ratio constitutes what is to our knowledge the sharpest spatial definition attained with freely propagating electromagnetic radiation. The sub-diffraction spots enable for the first time far-field fluorescence microscopy with resolution at the tens of nanometer scale, as demonstrated in images of membranes of bacillus megaterium.

Spatial correlations in the near field of random media.

Abstract: The conventional coherence theory suggests that the fields radiated by statistically homogeneous sources correlate over spatial regions of the order of the wavelength irrespective of the distance from the surface of the source. Contrary to these predictions, we show that the spatial correlations of optical fields in close proximity of highly scattering, randomly inhomogeneous media depend on this distance and, moreover, their extent can be significantly smaller than the wavelength. The contribution of evanescent fields is experimentally demonstrated and the coherence length in the near field is shown to relate to the coherence properties at the surface which are, in turn, determined by the structural characteristics of the random media.

Broadband near-field interference spectroscopy of metal nanoparticles using a femtosecond white-light continuum

Abstract: We demonstrate a new nanoscale spectroscopic technique that combines subwavelength near-field imaging with broadband interference spectroscopy. We apply this technique to study phase spectra of surface plasmons in individual gold nanoparticles and nanoparticle dimers. Collective plasmon oscillations in selected nanostructures are excited by a femtosecond white-light continuum transmitted through a subwavelength aperture. The interference spectra detected in the far field result from the coherent superposition of the aperture field and the secondary field re-emitted by the nanostructure. The analysis of these spectra allows us to accurately measure the positions and damping constants of single-nanostructure plasmon resonances.

Near-field optical probe

Abstract: The present invention has an object, in a near-field optical probe having a microscopic aperture to generate and/or scatter a near field, to obtain a near-field optical probe easy to be made in an array which increases the intensity of a near field to be generated and/or scattered and is adapted for use as an optical memory head. This near-field optical probe is arranged with a planar lens having microscopic lens on a flat surface substrate having an inverted conical or pyramidal hole formed therethrough such that its apex is made as the microscopic aperture, wherein a light source is further arranged thereon to introduce light to the planar lens. Because the arrangement is made such that the planar lens has a focal point positioned at the microscopic aperture, the light given by the light source can be efficiently collected to the microscopic aperture. Also, the above structure can be arrayed and mass produced using a silicon process, thus being adapted for use as an optical memory head.

Near-field-far-field transformation with spherical spiral scanning

Abstract: An efficient near-field-far-field transformation technique with spherical spiral scanning, which uses a minimum number of data, is proposed. A nonredundant sampling representation of the electromagnetic field on the spiral and a fast, accurate and stable interpolation algorithm are developed to this end. By choosing the elevation step of the spiral coincident with the sample spacing needed to interpolate the field on a meridian, it is possible to reconstruct the field at any point on the spherical surface. This allows the evaluation of the data required by a spherical near-field-far-field transformation. Numerical examples assessing the effectiveness of the proposed technique are reported.

A uniqueness theorem for an inverse electromagnetic scattering problem in inhomogeneous anisotropic media

Abstract: We show that the support of a (possibly) coated anisotropic media is uniquely determined by the electric far field patterns corresponding to incident timeharmonic electromagnetic plane waves with arbitrary polarization and direction. Our proof avoids the use of a fundamental solution to Maxwell’s equations in an anisotropic medium and instead relies on the well-posedness and regularity properties of solutions to an interior transmission problem for Maxwell’s equations.

Ridge waveguide as a near-field optical source

Abstract: We investigate the feasibility of using a ridge waveguide at optical frequencies as a near-field optical transducer, using the finite difference time domain method. The complete electromagnetic field picture of the ridge waveguide, in the absence and presence of the irradiated medium, is presented. A power efficiency of 7% and an optical spot with full width half maximum of 50 nm×80 nm is obtained in the medium. We show that impedance considerations play a major role in the transducer-medium optical coupling.

Propagation of the electromagnetic field in fully coated near-field optical probes

Abstract: Fully metal-coated near-field optical probes, based on a cantilever design, have been studied theoretically and experimentally. Numerical simulations prove that these structures allow nonzero modal emission of the electromagnetic field through a 60-nm-thick metallic layer, that is opaque when deposited on flat substrates. The far-field intensity patterns recorded experimentally correspond to the ones calculated for the fundamental and first excited LP modes. Moreover, this study demonstrates that a high confinement of the electromagnetic energy can be reached in the near-field, when illuminated with radially polarized light. Finally, it was verified that the confinement of the field depends on the volume of the probe apex.

Combined far‐ield operators in electromagnetic inverse scattering theory

Abstract: We consider the inverse scattering problem of determining the shape of a perfect conductor D from a knowledge of the scattered electromagnetic wave generated by a time-harmonic plane wave incident upon D. By using polarization effects we establish the validity of the linear sampling method for solving this problem that is valid for all positive values of the wave number. We also show that it suffices to consider incident directions and observation angles that are restricted to a limited aperture. Copyright © 2003 John Wiley & Sons, Ltd.

Reflection and transmission by reinforced concrete - numerical and asymptotic analysis

Abstract: To model the reflection and transmission properties of reinforced concrete, we develop a method of moments/Green's function (MoM/GF) model for a wire grid embedded in a lossy dielectric slab. Since the scattering by a wire grid is dominated by wires that are parallel to the incident electric field, a two-dimensional (2D) model with a grid of straight, parallel wires is used. The number of wires is finite, but can be large. The dielectric slab can be thin or thick, and it is of infinite transverse extent. The source and field points are in the near field. The wire currents are solved for by the MoM. Certain Green's functions for the dielectric slab are evaluated by two different methods: 1) exact numerical evaluation of Sommerfeld integrals (SI); 2) the high frequency asymptotic approximation, via the saddle point method, which yields the geometrical optics (GO) result. We explore the range dependence and angular dependence of the reflected and transmitted fields. The influence of surface waves and wire-wire interactions is also examined.

Optical near-field generator and recording apparatus using the optical near-field generator

Abstract: A near-field optical probe and optical near-field generator are provided. A problem of a probe having a scatterer in which optical near-field noises are generated at the parts other than for a point at which an intense optical near-field is generated, is solved. In one example of the probe, a surface of the parts except for a vertex of the scatterer at which the intense optical near-field is generated is etched so that an etching depth becomes not less than a penetration depth of the optical near-field. The probe facilitates control of noises when a sample is observed or recording marks are reproduced.

Three-dimensional orientation of single molecules observed by far- and near-field fluorescence microscopy

Abstract: We present a simple and straightforward method for determining absolute spatial orientations of transition dipole moments of single fluorescent molecules. Far-field polarization microscopy provides angles of the dipole moments projected in the plane of the sample. Optical field near total internal reflection surfaces has a strong component perpendicular to the sample and, for a given in-plane angle, provides unambiguous orientation of the molecular dipole moment. Experimentally, both excitation modes are alternated to monitor real-time conformational dynamics of tetramethylrhodamine molecules covalently attached to a quartz substrate.

MLFMA analysis of scattering from multiple targets in the presence of a half-space

Abstract: The multilevel fast multipole algorithm (MLFMA) is applied to the analysis of plane-wave scattering from multiple conducting and/or dielectric targets, of arbitrary shape, situated in the presence of a dielectric half-space. The multiple-target scattering problem is solved in an iterative fashion. In particular, the fields exciting each target are represented as the incident fields plus the scattered fields from all other targets. The scattered fields from each target are updated iteratively, until the induced currents on all targets have converged. The model is validated with an independent scattering algorithm, and results are presented for several example multitarget scattering scenarios.

Near-field optical investigation of three-dimensional photonic crystals.

Abstract: We show that the coupling of light from an external pointlike light source into a three-dimensional photonic crystal depends on the relative launching position with respect to the crystal lattice as well as on the frequency of light. The results are obtained with a near-field technique which is used to acquire optical information beyond the diffraction limit and to access optical details within the unit cell of the crystal. The experiments are performed at frequencies near the second-order L-gap. As a result, the changes in the shape of the near-field pattern are explained by the photonic properties of the crystal.

The influence of the measurement probe on the evaluation of electromagnetic fields

Abstract: In this paper, the influence of the measurement probe on the evaluation of the far and near field of an electromagnetic source is characterized. While measuring, the electromagnetic field will be disturbed by the measurement probes themselves. Therefore, not the true, free-space field but the disturbed field will be measured. The disturbance can not be fully taken into account by the calibration, especially in the near field. It is found that in the near field, these disturbances are much higher than in the far field for a large sensitive measurement probe. This paper will show that when the disturbance must be limited to a predefined value (e.g., 5%), a suitable measurement probe with maximum sensitivity can be selected. The characterization was performed using a numerical electromagnetic computational program.